For me it's very simple: NSFW can't have a general acceptable definition because it depends on culture, background and personal beliefs. There is no way for a collection of communities to have a common definition and even if they would have: enforcement and interpretation is still done by volunteers.
Therefore All is never safe for work unless I know that my tolerance is lower than all communities within lemmy AND I'm fine with an accidental penis or breast due to human error.
How about "what I want is beyond my comprehension"?
That's quite often my default...